The coating for a particulate magnetic recording disk includes magnetic particles, such as gamma Fe.sub.2 O.sub.3 particles, within an organic binder. Load bearing particles, such as Al.sub.2 O.sub.3 particles, may also form part of the coating to improve the durability of the coating. The binder typically includes heat-hardenable polymers (such as epoxy or phenolic resins or combinations thereof, polyurethanes or acrylates), solvents, dispersing agents, and flow agents. In addition, the binder may include certain substances, such as organic additives which may decompose during curing of the coating and leave micropores on the surface of the cured coating, the micropores serving as minute reservoirs for a liquid lubricant which is later applied to the cured disk coating.
In the manufacture of a particulate magnetic recording disk, the magnetic particles are typically placed first in a media mill or ball mill to break-up agglomerations and to further enhance dispersion of the particles. The magnetic particles are then mixed in the organic binder. The resulting formulation is applied as a coating to the disk substrate by pouring it onto the substrate while it is spinning. A magnetic field is applied to the coating to orient the magnetic particles within the coating so that the particles are aligned in the preferred direction. The disk is then placed in a curing oven to cross-link the polymers in the organic binder and thus harden the coating. During the curing process, some substances may decompose or evaporate to leave micropores in the cured coating. After curing, the disk coating is buffed and then washed to remove any debris, followed by an optional burnishing operation to remove any asperities. The final step in the manufacturing process is the application of a liquid lubricant to the disk. The lubricant is absorbed into the micropores in the coating to provide lubrication between the head and the disk.
While certain substances used in the binder for other purposes may inherently decompose or evaporate during curing to form the micropores, it is also known to add certain substances for the specific purpose of micropore formation. For example, German patent application No. DE 33 41 205, published May 17, 1984, discloses the addition to the binder of a volatile material which evaporates during curing to leave micropores for retention of a liquid lubricant. U.S. Pat. No. 4,546,035, assigned to the same assignee as this application, discloses the use of polymeric additives, such as polybutene, which thermally degrade during curing and which have high volatilization efficiency to produce microvoids in the coating surface. The use of decomposing additions which form the micropores in the disk coating surface is difficult to control. If there are not enough micropores or if they are not of sufficient size, the disk will be under-lubricated which may result in wear between the disk and the head. On the other hand, if there are too many micropores or if they are of too large a size, the disk will be over-lubricated which may make it difficult for the head to break loose from the disk at startup of the disk file.